The present invention relates generally to the production of superplastically formed, metal alloy structures and more particularly to the production of metallic structural panels superplastically formed from metal tubes contained within and diffusion bonded to a metallic pan, and the associated methods for producing such structures.
Superplasticity is the characteristic demonstrated by certain metals to develop unusually high tensile elongations with minimum necking when deformed within a limited temperature and strain rate range. This characteristic, peculiar to certain metal and metal alloys, has been known in the art as applied to the production of complex shapes. It is further known that at these same superplastic forming temperatures, the same materials can be diffusion bonded with the application of pressure at contacting surfaces.
The prior art relating to superplastic forming of metallic sandwich structures, as well as metallic structural panels, is exemplified by U.S. Pat. Nos. 4,217,397 and 4,304,821 (hereinafter the '397 and the '821 patents, respectively), both of which issued to Hayase et al. The disclosure of both the '397 and '821 patents are hereby incorporated by reference. The '397 patent discloses a four sheet metallic sandwich structure, while the '821 patent discloses the corresponding method of fabricating the structure disclosed in the '397 patent.
The '397 patent discloses a metallic sandwich structure in which metal worksheets are joined by an intermittent weld. The joined sheets are thereafter sealed by a continuous weld to form an expandable envelope. Following the placement of the joined sheets in a limiting structure, inert gas pressure is applied to the interior of the joined sheets as well as the interior of other worksheets if other worksheets are utilized so that the structure diffusion bonds to itself or other worksheets. The core configuration of the structure is determined by the intermittent weld pattern. Face sheets may by formed for the sandwich structure from one sheet of the joined sheets or by additional sheets which may be inserted in the limiting fixture prior to the envelope's expansion such that the envelope will be expanded against and diffusion bonded to the inserted face sheets.
The metallic sandwich structure disclosed by the '397 patent and its method of preparation disclosed by the '821 patent are of limited utility in several areas. For example, in many applications it is desirable to introduce coolant to the interior of a metallic sandwich structure to absorb the heat which has built up within the structure. However, the complex series of intermittent welds which join the interior metal worksheets restrict the ability of coolant to flow through the structure in any controlled manner. While it is possible to design a metal sandwich structure as disclosed in the '397 patent which does permit controlled coolant flow, it is an onerous task as the series of intermittent welds are necessarily interconnected in order to have an even pressure distribution upon fabrication. This interconnection of the intermittent welds results in multiple paths for coolant flow and ultimately uneven heat extraction.
The metallic sandwich structure disclosed by the '397 patent is also limited in the type of shapes which it may form and the thickness of the resulting structure. The structure of the '397 patent has not been suitable for tightly rounded shapes, such as the leading edge of an aircraft which requires a small radius of curvature. For the '397 patent to be utilized for highly curved structures, the weld nugget line of the structure must be placed in the center of the curve for best results. However, locating this weld nugget line in the curve's centerline is a difficult alignment process and may be virtually impossible with some potential weld configurations.
Additionally, the metal sandwich structure of the '397 patent is not easily manufacturable in thicknesses of less than one-half inch or greater than four inches. For structures which must be greater than four inches in width, the metal sheets forming the structure tend to be stretched too far and become thinned out. The resulting sandwich structure is therefore weak or may not form the pattern which was anticipated by the intermittent weld pattern. Alternatively, for thin structures less than one-half inch, a typical sandwich structure formed from four sheets may not diffusion bond as completely as desired. For example, the joined sheets in the region of an intermittent weld will expand upon the application of increased pressure and may not form tightly enough to one another near the weld so as to bond to the interior two sheets. Thus, the resulting sandwich structure may not be as unified or as strong as desired.
It would be desirable to develop a metallic structural panel through which coolant could flow to provide enhanced heat transfer capabilities. It would also be desirable for the metallic structural panel to be capable of being formed in shapes with complex curvatures and shapes requiring small radii of curvature. Furthermore, it would be desirable if the metallic structural panel were capable of being fabricated in both very thin and thick widths.